The arrival direction (φ, θ) of a wave can be estimated using an array antenna arranged in a planar manner, such as an circular array. Here, φ represents the azimuth in the same plane (xy plane) as the plane of an array antenna, and θ represents the elevation when the perpendicular direction to that plane is the z axis.
Further, when detecting each arriving wave component by demultiplexing a plurality of arriving waves having high correlation (coherent) into each arriving wave component, the spatial smoothing technique using a plurality of subarray antennas provided in positions that shift horizontally in space, is effective. This method realizes suppression of correlation between arriving waves by determining the arithmetic mean of correlation matrices obtained in each subarray antenna.
Patent Literature 1 and Non-Patent Literature 1 disclose a method of estimating the arrival direction of a wave while suppressing correlation between coherent waves by applying the spatial smoothing technique to the above-described planar array.
FIG. 1 shows an array antenna disclosed in Patent Literature 1. In FIG. 1, array antenna 10 is formed with five antennas 100-1 to 100-5 that are arranged on a plane. Further, array antenna 10 is configured with subarray antenna 101-1 formed with antennas 100-1 to 3, and subarray antenna 101-2 formed with antennas 100-3 to 5. That is, subarray antenna 101-1 and subarray antenna 101-2 share antenna 100-3.
As is clear from FIG. 1, when being shifted horizontally, subarray antenna 101-1 coincides with subarray antenna 101-2. Therefore, it is possible to employ the spatial smoothing technique for array antenna 10, and it is possible to suppress correlation between arriving waves as described above, by calculating correlation matrices for a reception signal in each of two subarray antennas 101 and determining the arithmetic mean of the obtained two correlation matrices. As a result of this, it is possible to accurately detect respective arrival directions of a plurality of arriving waves having high correlation.